1. Field of the Invention
The present invention relates to a method and an apparatus for securing a distance of a spark discharge gap of a spark plug, in particular, by means of image data processing based on a plurality of measurement lines.
2. Description of Related Art
A spark plug has generally a center electrode fitted through an insulator into a housing and a ground electrode fixed to the housing. The center electrode partly exposed out of the insulator faces the ground electrode with a predetermined distance of spark discharge gap therebetween.
To secure the predetermined distance of spark discharge gap, a conventional method or apparatus has steps or means of holding the spark plug, reducing the distance of the spark discharge gap by pressing the ground electrode, taking a picture of the spark discharge gap by a CCD camera as image data and processing the image data.
According to the method or apparatus mentioned above, a measurement is generally conducted along a center axis of the center electrode to detect the distance of the spark discharge gap. That is, as shown in FIG. 5A, the measurement is conducted along a measurement line J5 at a center portion (center axis) of the center electrode, in the spark plug in which the insulator J4 holding the inside center electrode J3 is assembled and fitted to the housing J2 holding the outside ground electrode J1.
However, when the insulator J4 is assembled to the housing J2, there is a fear that a center axis (a measurement line J6) of the insulator J4, that is, the center axis of the center electrode J3 is shifted from a center axis (the measurement line J5) of the housing J2, as shown in a dotted line of FIG. 5A. As the camera for taking the picture of the spark discharge gap is positioned with respect to the housing J2 as a reference position, the position relationship between the camera and the insulator J4 may be varied, as the case may be. Therefore, the measurement line for detecting the distance of the spark discharge gap is not always on the center axis of the center electrode and, when the center electrode J3 is slender, may locate outside the center electrode J3.
To cope with the problem mentioned above, there is a way that the position of the measurement line is corrected with respect to the center electrode as the reference position. However, only one of the measurement line is not sufficient to secure an accurate distance of the spark discharge gap.
For example, when the center electrode J3 has a protruding portion J7 such as a burr at an edge thereof, as shown in FIG. 5B, the spark discharge gap is practically on a line through the protruding portion J7, on which a minimum distance of the spark discharge gap is formed. However, as the measurement is made along the center axis (measurement line J5 in FIG. 5B) of the center electrode J3, the distance between the center portions of the center electrode J3 and the ground electrode J1 is incorrectly recognized as the distance of the spark discharge gap. In another words, there is a possibility that, even if an unusual shaped portion such as the burr mentioned above exists partly on the center electrode J3, the unusual shaped portion is overlooked.